In the formation of polyester-reinforced rubber articles, a widely-used bonding agent for bonding the polyester to the rubber is the "RFL dip", comprising an aqueous mixture of a rubber latex and a condensation product of resorcinol and formaldehyde. The polyester fibre is dipped in this mixture, baked for a short period of time, brought into contact with a vulcanisable rubber composition, then the whole is subjected to vulcanisation conditions, the polyester becoming bonded to the rubber during the vulcanisation process.
In attempts to improve the bond strength between the polyester and the rubber, various proposals have been made to use this standard "RFL" dip treatment in combination with other treatments. One such method is described in UK Patent Specification No. 1140528 or the corresponding patents in other countries, e.g. U.S. Pat. No. 3660202 or Japanese Pat. No. 632024.
UK Specification No. 1140528 describes resinous condensation products of formaldehyde with certain monohydric phenols and certain dihydric phenols and their use in the bonding of polyester articles to rubber. For this purpose the condensation products are dissolved in aqueous ammonia and used in the form of the ammonium salts in conjunction with rubber latex and a formaldehyde-resorcinol condensation product, either in a two-stage or one-stage dipping process including a baking step.
In commercial practice it is found that the use of ammonia in this connection gives rise to objectionable fuming during all stages from opening the container to baking the film. It has now been found that certain amines can be substituted for the ammonia and the resultant salts used in a similar way to overcome the fume problem and to give additional advantages.
Thus according to the present invention there are provided salts of alkanolamines having alkanol residues of C.sub.2 to C.sub.3 and fusible substances having a molecular chain containing at least one each of the groups X--, --Y-- and --Z, connected by methylene groups, a molecular weight of at least 300, melting in the range 10.degree. to 300.degree. C. and having a solubility of at least 1% by weight in N aqueous ammonia at 25.degree. C., wherein X and Z are the same of different divalent radicals of resorcinol or substituted resorcinols and Y is a monovalent radical of a monobromo- or monochlor-monohydroxybenzene, an alkylmonohydroxybenzene wherein the alkyl group is C.sub.1 -C.sub.5 or a dichloro-, dibromo- or C.sub.1 -C.sub.5 dialkyl-monohydroxybenzene in which at least two of the 2,4 and 6 positions, relative to the hydroxy group, are unsubstituted.
The alkanolamines used to prepare the above salts are preferably saturated, linear, branched or cyclic compounds containing at least one --N.C.sub.n H.sub.2n group where n is 2 or 3, having a molecular weight up to about 300, and a boiling point of 150.degree. C. to 350.degree. C., preferably 165.degree.-280.degree. C.
As examples of these, there may be mentioned:
monoethanolamine PA1 diethanolamine PA1 triethanolamine PA1 1-amino-2-hydroxypropane PA1 1-amino-2,3-dihydroxypropane PA1 N-(2-hydroxyethyl)-ethylenediamine PA1 O-(2-aminoethyl)ethylene glycol PA1 N,n-diethylaminoethanol PA1 N-methyldiethanolamine PA1 N,n'-bis(hydroxyethyl)piperazine.
The preferred alkanolamine is monoethanolamine.
The fusible substances defined in claim 1 of UK Specification No. 1140528 are condensation products of formaldehyde, certain monohydric phenols and certain dihydric phenols. A substantial proportion of the products are believed to be compounds of the formula EQU X--CH.sub.2 --(Y--CH.sub.2).sub.n --X
where X is dihydroxybenzene radical, Y is a monohydroxybenzene radical and n is an integer. Such compounds can form a series of salts because of the presence of 5 or more hydroxyl groups, and depending on the number of these neutralised by the alkanolamine.
The chemical constitution of the fusible substance can be varied by altering the resorcinol or substituted resorcinol and monohydroxybenzene used, also their ratio to each other; however, this has relatively small effect upon the bonding action of the product. The preferred resorcinol is resorcinol itself and the preferred monohydroxybenzenes are p-cresol or above all p-chlorophenol.
When using resorcinol and p-chlorophenol, particularly useful condensation products are those using from 1.0 to 2.0 moles of resorcinol per mole the p-chlorophenol. For economic rather than technical reasons it is preferable to use those obtained at the lower end of this range, e.g. from 1.0 to 1.5 moles of resorcinol per mole of p-chlorophenol.
In the present invention, the preferred salts are those containing 0.4 to 0.8 equivalents of the alkanolamine per hydroxyl group of the condensation product, that is to say that on average about two to four of the hydroxyl groups of pentahydric compounds (n=1) of the kind described above are converted to the salt form.
The alkanolamine salts can be prepared by adding the appropriate amount of alkanolamine to an aqueous suspension of the condensation product; suitable dispersions can contain from 1% to 50% by weight of the condensation product, but concentrations of from 10% to 25% by weight are preferred. Alternatively, the resin can be dissolved in an alkanolamine solution of suitable strength e.g. a solution of 2N or 3N ethanolamine is found to be suitable for preparing a 20% solution of the condensation product.
The resultant solutions can be converted to the dry salt by evaporation at moderate temperatures which avoid loss of the alkanolamine by vapourisation. The temperature which can be used in this connection will vary according to the boiling point of the alkanolamine used but is preferably at least 20.degree. C. below the boiling point thereof.
However, for practical use in the bonding process of UK Specification No. 1140528, the aqueous solutions of resin can be stored for use directly in the bonding process. When used in this way, not only is the fume problem associated with ammonia overcome, but additional advantages are found in a greater ease of cleaning out the dip-baths and associated apparatus and frequently slightly higher bond strengths are obtained than from an ammoniacal solution. There appears to be little deleterious effect on the tensile strength of polyester cords as compared with the use of ammonia.
The use of the alkanolamine salts for improving the bonding of polyester shaped articles to rubber forms a further feature of the invention.
For this purpose, the salt may be added to a resorcinol-formaldehyde-latex dip commonly-used as described above, or may be applied to the polyester in a separate dipping operation, using an aqueous solution of the salt. The polyester coated with both salt and RFL dip is baked at a temperature from about 200.degree. up to about 250.degree. C., then brought into contact with an unvulcanised rubber mass and the whole is subjected to vulcanisation in the normal way.